Are there still travel agents?
I book everything online and have used Travelocity the last two times. They have the easiest website to navigate and give you the exact total price upfront. Also, they show which cabins are still available so you can choose the one you want. Finally, they currently have a MasterCard special that saved me $150 on the last booking--I couldn't find that with any other service.
When there was a price drop after booking my first cruise with Travelocity I called them and they were able to get me a rebate. Normally, it only takes a couple of minutes to reach their telephone reps and, so far at least, they have all been very knowledgeable and helpful.